1. Field of the Invention
The present invention relates to an encoder that detects a position of an object.
2. Description of the Related Art
Previously, there is an optical apparatus such as a lens apparatus or a camera that has an autofocus (AF) function. The AF function is achieved by a focus detection system that detects an in-focus state of an object, a lens drive system that moves a focus lens so as to form an object image on an image pickup element, and an AF control system that controls the focus detection system and the lens drive system. The focus detection system performs focus detection by a phase difference method, a contrast method, or the like. The lens drive system is provided with an encoder that detects a position of the focus lens. The AF control system controls a motion of the focus lens based on an output of the encoder.
Japanese Patent Laid-Open No. 2007-47652 discloses a configuration of a drive system to improve detection accuracy of a position detection encoder. Japanese Patent Laid-Open No. 2009-169052 discloses a configuration of including a ball and a ball race so that a rotation transfer ring of a lens drive system is held rotatably at a constant position.
Recently, the demand of a higher-accuracy AF is increasing in order to obtain an image which is focused on an object with higher accuracy, and therefore an encoder capable of performing position detection with higher accuracy and higher resolution is required. However, in Japanese Patent Laid-Open No. 2007-47652, a roller ring to drive a lens is held with a fitting backlash with respect to a unit body. Therefore, high accuracy of the position detection is prevented due to an unnecessary motion in the fitting backlash. In Japanese Patent Laid-Open No. 2009-169052, the rotation transfer ring is held without a backlash so as to be rotatable at the constant position by using the ball and the ball race. However, a plurality of annular ball races are necessary and it is difficult to improve a space efficiency.